1. Field of the Invention
This invention relates to filters in general and more particularly to microwave hairpin filters that have improved low frequency stop band and pass band performance.
2. Description of Related Art
Many different types of filters are known for the processing of electrical signals. For example, in communications applications, such as for microwave frequencies, it is desirable to filter out small individual pass bands. This allows a fixed frequency spectrum to be divided into a large number of bands. In order to select certain bandwidth frequencies, the bandwidth must be reduced by rejecting unwanted frequencies above and below the desired bandwidth. The objective of a filter is to have a minimum loss of the frequencies in the desired bandwidth, (called the pass band), with significant losses of the unwanted frequencies below and above the desired pass band of frequencies. The unwanted low frequency bandwidths are referred to as low frequency stop band. The unwanted high frequency bandwidths are referred to as high frequency stop band.
In certain applications, greater rejection of the low and high frequency stop bands are necessary than a single resonator filter can achieve. For greater rejection, additional resonators must be added to the filter. Typically, the greater the number of resonators, the greater the rejection of unwanted high and low frequencies. However, adding additional resonators also increases insertion loss in the pass band and also increases the physical size of the filter. The additional resonators add complexity and take up more space on a printed circuit board.
A well known prior art filter is shown in FIG. 1. FIG. 1 is a hairpin micro-strip filter. Filter 20 has a substrate 32 with a top surface 32A and bottom surface 32B. An input coupling element 34, a U shaped resonator 50 and an output coupling element 40 are located on top surface 32A. Input coupling element 34 has an input pad 35 and coupling line 36. U shaped resonator 50 has a closed end 52 and an open end 54 Output coupling element 40 has a pad 41 and coupling line 42. A gap 56 is located between input coupling element 34 and resonator 50. A gap 58 is located between output coupling element 40 and resonator 50. The substrate can be ceramic or a soft printed circuit board. The resonator and coupling elements would typically be etched copper printed circuit lines. The input coupling element, output coupling element and resonator are electromagnetically coupled as is known in the art.
The filter of FIG. 1 reduces the amount of space needed for multiple resonators. As each resonator is added, the hairpin configuration condenses the physical size by utilizing side by side coupling.
Referring to FIG. 2, a three resonator prior art filter 25 having three hairpin resonators mounted side by side is shown. Filter 25 is similar to filter 20 except that three resonators 50 are mounted side by side between the input and output coupling elements. Gaps 60 separate the resonators.
Certain applications place a greater requirement on rejecting the low frequency stop band relative to the high frequency stop band. For example, in filtering a signal after utilizing frequency doublers or frequency multipliers. The prior art hairpin filters do not provide adequate sub-harmonic suppression with a given quantity of resonators. Further, the prior art filters require multiple resonators which take up excessive printed circuit boards space.
While various band pass filters have previously been used, they have suffered from not having enough rejection in the low stop band, excessive loss in the pass band, being expensive to produce and requiring excessive circuit board space.
A current unmet need exists for an improved filter that is compact, has greater suppression, improved low frequency stop band performance, minimum loss in the pass band and is readily manufactured at low cost.
It is a feature of the invention to provide a hairpin filter that has improved low frequency stop band performance and improved pass band performance.
Another feature of the invention is to provide a hairpin filter that is more manufacturable at lower cost.
Another feature of the invention to provide a filter that includes a dielectric substrate. The dielectric substrate has a top and bottom surface. A hairpin resonator is mounted to the top surface. The resonator has an open end and a closed end. An input coupling element is located adjacent to and is communicated with the resonator. An output coupling element is located adjacent to and is communicated with the resonator. A first inductive element is connected to the resonator. A second inductive element is connected to the input coupling element. A third inductive element is connected to the output coupling element.